The present invention is concerned with inertia type decelerometer switches and particularly such decelerometer switches in which an electrically conductive liquid is confined within a glass tube or the like for the purpose of controlling energization of an electrical circuit including a pair of contact terminals extending into the tube.
One application of a decelerometer switch of this type is in a brake assuring circuit, such as is generally employed on rapid transit type railway vehicles operating in speed restrictive territory under automatic control. The decelerometer switch is used in an external brake control loop to control a back-up, emergency type brake application in the event a minimum deceleration rate fails to materialize during an automatic, speed control initiated brake application intended to reduce the vehicle speed to the speed authorized for the particular speed zone in which the vehicle is operating. In this sense, the minimum deceleration rate at which the decelerometer switch is set to operate is necessarily less than the rate expected to be realized by the automatically initiated brake application. In the event this minimum deceleration rate is not realized, it must be assumed that the automatic controls are malfunctioning and an emergency brake application is therefore necessitated.
It will be appreciated that in order to provide the railroad properties with the latitude to vary this minimum deceleration rate, as may be dictated by their respective operating conditions, an adjustable deceleration range is desirable for the decelerometer switch. However, if the lower limit of this adjustable deceleration range is set too low, the deceleration switch may fail to initiate an emergency brake application until too late. This, of course, dictates that the lower limit of the adjustable deceleration rate be positively maintained at the proper setting by some locking means, in order to prevent tampering by unauthorized personnel and to accordingly avoid an obviously unsafe operating condition of the decelerometer switch. However, locking-in this lower set point of the adjustable deceleration range prevents mercury/contact closure from being used as a reference point and thus requires a different procedure for installing the deceleration switch in the field to obtain switch operation according to the factory calibrated deceleration setting.